Banquet chefs charge $28 per guest.
This means if:
there is 1 guest, they charge $28
there are 2 guests, they charge $28 x 2 = $56
there are 3 guests, they charge $28 x 3 = $84
there are 4 guests, they charge $28 x 4 = $112
Thus, we can generalize that if they have n guests,
they charge $28 x n = $28n
In addition to this charge per each guest, Banquet Chefs charge $590 for service fee.
Thus, if Barbara is to choose Banquet chefs, the fees she ought to pay is given by:

If she goes for Cece's catering, she has to pay $31 for each guest.
This means that if:
there is 1 guest, they charge $31
there are 2 guests, they charge $31 x 2 = $62
there are 3 guests, they charge $31 x 3 = $93
Thus, we can generalize that if Barbara invites n guests then she has to pay:
$31 x n = $31n
However, the question also says Cece's catering charges extra $425 for service fee.
Therefore, the total amount Barbara would pay is modelled by the equation:

Now, the question says that 20 guests attended the event. This means n = 20
To know how much Barbara has to pay for either Banquet chefs or Cece's catering, we simply substitute n= 20 into both equations respectively then pick the one with the lower fees.
This is done below:
Banquet Chefs:

For Banquet chefs, Barbara would pay $1150 for 20 guests.
Now, let us calculate for Cece's catering.
Cece's catering:

For Cece's catering, Barbara would pay $1045 for 20 guests.
Therefore, Barbara should choose Cece's catering because they charge $1045 for 20 guests which is lower than Banquet chefs who charge $1150